Light source with selectable intensity bandwidth

ABSTRACT

A light source having a selectable intensity bandwith comprising a lamp, a photosensitive element responsive to the lamp, switching means responsive to the photosensitive element for energizing the lamp below a first selected intensity and for deenergizing the lamp above a second selected intensity, the first and second selected intensities defining the intensity bandwith.

United States Patent 11 1 l 1 11 80,340

Munro Dec. 18, 1973 LIGHT SOURCE WITH SELECTABLE 3.431.464 3/1969 Brischnik 315/151 x ENS BANDWIDTH 3,456,155 7/1969 Buchananm. 315/151 3,475,676 l0/1969 Nutson 3l5/l56 X [75] Inventor: William Munro, Taunton, Mass.

[73] Assignee: Analog Monitors, lnc., Winthrop, Primary Examinere'eNathan Kaufman Mass. A!t0rneyJohn M. Brandt [22] Filed: June 18, 1971 ABSTRACT [2]] Appl' 154297 A light source having a selectable intensity bandwith comprising a lamp, a photosensitive element respon- [52] U.S. CI. 315/151, 315/156 sive to the lamp, Switching means r p i to th [51] Int. Cl. 1105b 39/04 ph n i i m n r n rg zing th l mp below [58] Field of Search 315/ 151, 156 a fi eleCted intensity and for de-energizing the lamp above a second selected intensity, the first and [56] References Cited second selected intensities defining the intensity band- UNITED STATES PATENTS 3,473,084 10/1969 Dodge 315/151 4 Claims, 2 Drawing Figures PAIENTEIJ 8I975 I I 3.780.340

LAMP INTENSITY OR INTENSITY BANDWIDTH ILLUMINATION LEVEL I I i I I INTENSITY I I MEAN I I I I K/I I I. I, j I I I I I I I I I I I I I I I I I I LAMP I I I I VOLTAGE I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I O 2 5 1 4 5 6 7 8 9 IO II TIME INVENTOR WILLIAM MUNRO ATTORNEY LIGHT SOURCE WITH SELECTABLE INTENSITY BANDWIDTH BACKGROUND OF THE INVENTION 1. Field of the Invention The invention resides in the field of light sources and belongs more particularly to the category of those sources having a self-regulating intensity control.

2. Description of the Prior Art The art of self-regulating light sources utilizing a photosensitive element in a feed back loop is well developed. Prior art devices fall generally into two categories, i.e.,'those which modulate the power delivered to the lamp and those which switch power totally on and off.

For example, United States Pat. Nos. 2,823,301 by Stevens and 2,477,646 by Perlow both disclose light intensity regulating systems wherein the grid of a vacuum tube is biased by the output of a photoresistor to control plate current which in turn regulates the power delivered to a lamp.

Two additional Pat. Nos. 3,473,084 by Dodge, and 3,483,428 by LaPlante illustrate the use of a photosen sitive element in a feedback system to control a switch for turning lamp power on or off. More specifically, the Dodge patent discloses a circuit in which power is turned on automatically by a series of pulses from a relaxation oscillator and switched off when a pre-selected intensity is reached as sensed by a photoresistor.

The LaPlante patent presents a current in which the firing angle of a silicon control rectifier in a power source for an arc lamp is controlled by the conductance of a photoresistor. The source in this arrangement is turned on when the output intensity of the lamp falls below a preselected level.

Among other advantages, those devices which employ switching techniques consume substantially less power in the feed back monitoring and control circuit than those which modulate the source power. The invention described below may be classed as a switching device, the design of which presents significant improvements in the art as known to the inventor.

SUMMARY OF THE INVENTION The invention essentially comprises a light source having a selected intensity bandwidth which source includes associated feedback circuitry for controlling and maintaining the bandwidth.

More specifically, a photosensitive element is placed in proximity to the lamp of the source which element controls circuitry for switching lamp power on when the lamp intensity falls below a selected level and for switching power off when the intensity rises above a second selected level. The two levels define the intensity bandwidth.

The photosensitive element is preferably a photoresistor whose conductance varies proportionately with the amount of light falling on the resistor. The resistor is placed in one arm of a bridge circuit which is unbalanced in accordance with changes in light intensity of the lamp.

In the preferred embodiment, the inverting and noninverting inputs of an operational amplifier are connected across the bridge and the output of the amplifier used to bias the base of a transistor which acts to switch power on and off to the lamp. The reaction time of the operational amplifier may be controlled in a number of ways, for example, by providing positive feedback to the noninverting input such that in effect a time lag is introduced into the operation of the switching transistor. During the lag, the lamp intensity will increase or decrease to a level dictated by the characteristics of the lamp filament.

Thus the intensity bandwidth is widened or narrowed according to the time lag of the amplifier which essentially controls the time lag of the switching of the lamp. It will be recognized that one extremely useful purpose of the invention is to produce a light intensity output of narrow bandwidth, that is, one which is essentially constant as was the object of the prior art devices referred to above. However, an electrical switching system which oscillates too quickly may produce undesirable radiation which will interfere with equipment being used in conjunction with the light source or which will interfere with other electrical devices not associated with the source.

Therefore a highly desirable feature of this light source is an intensity bandwidth which may be varied and selected to achieve optimal matching with associated equipment.

Other features and advantages of the invention will become more apparent from the following specification when read with the accompanying drawing.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a preferred embodiment of the invention; and

FIG. 2 is a graphical representation relating pertinent parameters of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a battery 10 connected to one side of lamp 12 by line 14 and the other side by line 16 through transistor 18. Switch 19 activates the entire circuit.

Photoresistor 20 forms one leg of a bridge circuit in conjunction with resistor 22. Variable resistor 24 forms a second leg in conjunction with resistor 26 and is used to set the mean intensity of the bandwidth. This resistor is shown as variable but may be fixed when a particular mean is preselected. Resistors 28 and 30 form the opposite legs of the bridge and are connected to the noninverting input 29 of operational amplifier 32 at their junction 34. The junction of resistors 22 and 26 is connected to the inverting input 31 of amplifier 32.

Power is fed to the amplifier through lines 36 and 38 connected to lines 14 and 16. The output 39 of the amplifier is connected to the base of transitor 18 through line 40. Resistor 44 is connected between the output of the amplifier and the noninverting input. The resistor as shown is variable but may be fixed when a particular bandwidth is preselected. Resistor 44 provides a limited amount of positive feedback and controls the time lag as will be explained below.

In operation the circuit is activated by closing switch 19. Variable resistor 24 may be presumed to be set at a value which will yield a desired mean intensity but which may be varied at will.

Photoresistor 20 receives no illumination from lamp 12 resulting in low conductivity or high resistance and a corresponding imbalance in the bridge circuit. Negative voltage from point 27 is applied to the inverting input of amplifier 32 resulting in a large positive voltage at output 39. The voltage at the base of transistor 18 is thereby raised and the transistor conducts providing power flow through lamp 12. Depending upon the value of resistor 44, some positive voltage is applied to the noninverting input of amplifier 32. The reference point of the amplifier, zero at the beginning since resistors 28 and 30 are identical and form opposite arms of the bridge circuit, is shifted slightly positive increasing the output of the amplifier. The effect is to further unbalance the bridge by providing positive feedback to the noninverting terminal of the amplifier.

The lamp intensity will continue to rise until the photoresistor is illuminated sufficiently to conduct an amount to balance the bridge. The resistance of the photoresistor is now equal to the set value of resistor 24. However, the positive feedback voltage must be overcome before zero voltage appears across the input terminals of the amplifier and the amplifier output is reduced to zero. Therefore, power will continue to flow to the lamp for a slight amount of time beyond the point at which the bridge arms composed of resistors 20 and 22 and 26 and 24 balance.

The amount of positive feedback controls the time lag which ensues before the transistor is switched off by the amplifier. During that time the intensity rises a certain amount above the mean to provide an upper intensity level.

When the amplifier input differential is reduced to zero, the amplifier produces no more output, the transistor ceases to conduct, lamp power is shut off, and the lamp intensity begins to decline.

The reverse of that described above will now occur. Positive voltage will appear'at the inverting input, negative voltage at the amplifier output, and a slightly negative reference voltage at the noninverting input. The transistor will remain switched off and the lamp intensity will fall at a rate determined by the physical characteristics of the lamp filament. The intensity will fall below the mean since a certain amount of negative voltage must appear at the inverting input before the negative voltage in the positive feedback loop is overcome.

The circuit will thus swing back and forth providing an intensity bandwidth which may be precisely selected by choosing the value of the resistance in the positive feedback loop. A table of component values and/or types which may be used in constructing the circuit is given below.

Value 44 l mcgohm Photo cell 20 Clairex CL-705-M Lamp 12 G.E. No. 67

Operational Amplifier 32 T.l. SN 72709N Referring to FIG. 2, a graph of lamp intensity and lamp power is plotted with respect to time. At time 0 when the circuit is energized, lamp power is applied and the lamp intensity rises. At time 1, the mean intensity is passed but power remains switched on due to the time lag introduced bythe feedback loop. At time 2 the feedback voltages is overcome by an overbalance due to further illumination of the photoresistor and a rise in conductivity. Power is switched off and illumination drifts downward through the mean at time 3 until feedback is again overcome and voltage is switched on at time 4. Time points 5 and 9 are analogous to point 1, points 6 and 10 to point 2, and points 7 and l l, to point 3.

What is claimed is:

l. A light source having a selectable intensity bandwidth comprising in combination:

a. a lamp;

b. a power supply for energizing said lamp;

c. electrical switching means connected between said power supply and said lamp for switching power on to said lamp below a first selected light intensity and for switching power off to said lamp above a second selected light intensity, said first and second intensities defining said intensity bandwidth, said switching means comprising:

1. an electrical resistance bridge having said power supply connected across its input;

2. a photosensitive resistance element responsive to light emitting from said lamp forming one arm of said bridge;

3. an operational amplifier having its input connected across the output of said bridge;

4. a switching transistor having its base connected to the output of said amplifier and its collector to said lamp; and

5. a resistance feedback loop between the output of said amplifier and the noninverting input of said amplifier for establishing said intensity bandwidth.

2. The apparatus of claim 1 wherein said resistance feedback loop comprises a variable resistance for varying said intensity bandwidth.

3. The apparatus of claim 1 wherein one resistance arm of said bridge is a variable resistance for varying the mean level of said intensity bandwidth.

4. The apparatus of claim 2 wherein one resistance arm of said bridge is a variable resistance for varying the mean level of said intensity bandwidth. 

1. A light source having a selectable intensity bandwidth comprising in combination: a. a lamp; b. a power supply for energizing said lamp; c. electrical switching means connected between said power supply and said lamp for switching power on to said lamp below a first selected light intensity and for switching power off to said lamp above a second selected light intensity, said first and second intensities defining said intensity bandwidth, said switching means comprising:
 1. an electrical resistance bridge having said power supply connected across its input;
 2. a photosensitive resistance element responsive to light emitting from said lamp forming one arm of said bridge;
 3. an operational amplifier having its input connected across the output of said bridge;
 4. a switching transistor having its base connected to the output of said amplifier and its collector to said lamp; and
 5. a resistance feedback loop between the output of said amplifier and the noninverting input of said amplifier for establishing said intensity bandwidth.
 2. a photosensitive resistance element responsive to light emitting from said lamp forming one arm of said bridge;
 2. The apparatus of claim 1 wherein said resistance feedback loop comprises a variable resistance for varying said intensity bandwidth.
 3. The apparatus of claim 1 wherein one resistance arm of said bridge is a variable resistance for varying the mean level of said intensity bandwidth.
 3. an operational amplifier having its input connected across the output of said bridge;
 4. a switching transistor having its base connected to the output of said amplifier and its collector to said lamp; and
 4. The apparatus of claim 2 wherein one resistance arm of said bridge is a variable resistance for varying the mean level of said intensity bandwidth.
 5. a resistance feedback loop between the output of said amplifier and the noninverting input of said amplifier for establishing said intensity bandwidth. 